Shift workers had a significantly higher risk of myocardial infarction, stroke, and coronary events compared with the general population, a review of published data showed.
The magnitude of the increased risk ranged from 5% for ischemic stroke to almost 25% for MI and coronary events. The risk persisted across different work shifts, with the exception of night-shift workers.
Adjustment for smoking or socioeconomic status did not affect the results, as reported online in BMJ.
"The increased risk of vascular disease apparent in shift workers, regardless of its explanation, suggests that people who do shift work should be vigilant about risk factor modification," David C. Hackam, MD, of Western University in London, Ontario, and co-authors wrote in conclusion.
"Screening programs of modifiable risk factors in shift workers have yielded substantial burdens of treatable risk factors, including dyslipidemia, smoking, glucose intolerance, and hypertension. Shift workers should be educated about cardiovascular symptoms in an effort to forestall or avert the earliest clinical manifestations of disease.
"Evidence also exists in the literature to suggest that modification and rationalization of shift schedules may yield dividends in terms of healthier, more productive workers; however, the long-term effects of these alterations on vascular outcomes remain unknown," they wrote.
Shift work has a well-documented potential to disrupt circadian rhythm, sleep, and work-life balance. However, the association with vascular disease remains controversial, characterized by conflicting data from multiple studies.
Previous efforts to assess associations between shift work and vascular risk have become outdated or did not use valid methodology, the authors noted in their introduction. The uncertainties surrounding the association provided a rationale for another literature review and analysis of data.
The authors examined multiple bibliographic databases, interviewed authorities in the field, and reviewed reference lists for published articles and guidelines. They rated study quality by means of the Downs and Black scale for observational studies.
The review identified 34 studies and a total of 2,011,935 adults. The studies consisted of 11 prospective cohorts, 13 retrospective cohorts, and 10 case-control analyses. Shift schedules represented in the studies were evening shift (four studies), mixed schedules (11), night shift (nine), rotating shifts (10), and irregular or unspecified shifts (six).
Control populations consisted of nonshift day workers in 30 studies, and the remaining four investigations compared shift workers to the general population.
The primary outcomes were MI, ischemic stroke, and any coronary event. Outcomes examined in the studies were MI in 10 cases, any coronary event in 28, and ischemic stroke in two.
A pooled random-effects analysis showed that shift work was associated with a 23% higher risk of MI (RR 1.23, 95% CI 1.15 to 1.31), a 24% higher risk of coronary events (RR 1.24, 95% 1.10 to 1.39), and a 5% higher risk of ischemic stroke (RR 1.05, 95% CI 1.01 to 1.09).
Comparison of adjusted and unadjusted pooled risk ratios showed similar results.
None of the studies included heart failure or hemorrhagic stroke among the outcomes of interest. However, cardiovascular events were evaluated in five studies, coronary mortality in nine, cerebrovascular mortality in four, cardiovascular mortality in five, and all-cause mortality in eight. Risk ratios exceeded 1.0 for all associations with shift work.
Pooled results did not differ significantly between studies that ranked higher or lower in terms of Downs and Black scores.
The authors acknowledged several limitations of their study: variation in outcome definitions across studies, variable approaches to statistical adjustment, a wide variety of cardiovascular conditions included under the categorical umbrella of "cardiovascular events," and few studies that assessed the endpoints of ischemic stroke and cerebrovascular mortality.
"Notwithstanding these limitations, we have identified an epidemiological association between shift work and vascular events," they wrote. "Unfortunately, observational studies such as those synthesized here cannot definitively prove causality.
"However, other studies have noted that shift workers have higher rates of dyslipidemia, metabolic syndrome, hypertension, and diabetes. Even a single overnight shift is enough to increase blood pressure and impair variability of heart rate."
Primary source: BMJ